Vertical stringing

[txjm]

Ok, I finally got my version of the 7.62x40 20" barrel to function. It even locks after the last round. Clipped a coil, lightened the buffer and opened the port to .125".
Now, what is wrong when an AR vertically strings groups? I am shooting a 20"+ take off remington barrel that I cut threads on, a high rise upper and a free float handguard. I am shooting on bags. I am shooting less than .75" groups with my 223 upper. I am getting a perfectly vertical group with this one. Maybe .75" x 2" maybe .75 x 2.5". I have gotten some really awesome 3 shot groups but if I keep shooting the groups alway end up stringing vertically.
I seem to remember somewhere I read that it may have something to do with the torque on the barrel nut being too loose or too tight??? Any suggestions??
Chip

[Spook]

assuming your powder charges are precisely equal... use slower powder or shorten the barrel

[txjm]

Originally Posted by Spook

shorten the barrel

Youre killing me. I love the velocity Im getting and I just got it to function. I know the shorter stiffer barrels are better in some cases but can you think of anything else? Is it harmonics? I don't see being able to change powder. Im pretty limited on bullets. Muzzel brake / weight might change harmonics??
Chip

[Alleycat]

A slower powder will not necessarily drop velocity. Have you checked the velocity for consistency?

[txjm]

Originally Posted by Alleycat

A slower powder will not necessarily drop velocity. Have you checked the velocity for consistency?

I do need to get my chrono back out. The rifle now functions consistently to check that kinda thing. I haven't heard much in the way of effective powders other than 1680 in this round. I did try 4198 and didn't get the velocities. I do fireform with h110 but it wont cycle the rifle reliably. I'll get the chrono out and check SD.
Chip

[kurtz]

glad to hear you got your 20" running.....I've had pretty good luck with 1680 and H4227 in my 20" 40 rifle length sytem....usually keep the velocity between 2,500 and 2,600 fps with 125s.....

most of the stuff I've messed with issues of vertical stringing was traced back to shot to shot velocity variance rather than something mechanical, but it's a good idea to check the system as a whole....

velocity variance can be caused by many things: variances in cartridge length can cause pressure swings in land jump and burn rate (especially if using compressed loads), neck tension, charge repeatability, chamber heat soak from shot to shot, barrel / bore heat changes, chamber and bore fouling.....the more you shoot there is an incrimentation of heat build up in the upper half of the mag (sometimes to the point that the copper will turn rose colored, can cause powder burn swings)....several of the guys that I know that shoot DI ARs for groups single load at a set cadence to minimize mag and chamber soak temp issues....

[txjm]

Originally Posted by kurtz

....the more you shoot there is an incrimentation of heat build up in the upper half of the mag (sometimes to the point that the copper will turn rose colored, can cause powder burn swings)....several of the guys that I know that shoot DI ARs for groups single load at a set cadence to minimize mag and chamber soak temp issues....

I have had a quite a few that the first 2 were touching then the next would be high but nothing I can count on. I will get the chrono back out and see if I am getting a velocity diff that I can correlate.

Thanks kurtz
The 20 is awesome now that it is fully functional. Pigs this weekend.
Chip

[Medic650]

Where can I find more info on the 7.62x40? I hadn't heard of ot except on this forum.

[Fudmottin]

Sounds like a 223 case that isn't cut down as much as for 7.62x34, AKA 300-221.

As for the solution, I think the usual suspects have been mentioned.

[txjm]

"Where can I find more info on the 7.62x40? I hadn't heard of ot except on this forum."
I believe Kurtz is the man on the 7.62x40. I think he has a really long string on ar15, in the Variants section I think.
I think you can get any info that you would want from Kurtz. I think I saw info on barrels and dies also.
Mine is nothing more than running a 300w reamer in deep enough to seat a 125 comfortably at mag length. I was chambering for the 300w and noticed that the reamer was on the small side and just decided to run it in deeper to see how it would work. Ive had no issues. The brass is holding up well and it feeds good. I think my shoulder is farther back than kurtz's 40 and he has a little more case capacity. I didn't have much to lose, I was working with a barrel someone gave me left over from a remington donor action. I may still go back and see if I can get ream this chamber again with a 7.62x40 reamer.
If I was to do it again I would go with a 762x40 because of the availability of dies. I am messing around with multiple dies to resize.
I like my 300w but the 762x40 has some advantages on dedicated supersonic uppers. It has become my pig rifle.

Link to 762x40 string
http://www.ar15.com/forums/topic.htm...f=121&t=382761